A tractor, bulldozer or similar off-highway vehicle has an unsprung chassis, and therefore the operator of such a vehicle would be subjected to severe jolting and shaking if the vehicle seat were not supported on the chassis by means of a resilient shock absorbing suspension. Among the cushioning devices heretofore proposed for resilient seat support are those comprising metal springs, such as disclosed in U.S. Pat. Nos. 2,840,140 and 3,787,025, and those comprising air springs, such as disclosed in U.S. Pat. Nos. 3,913,975 and 3,994,469.
A resilient seat suspension must permit the seat to have a certain amount of up and down motion with respect to the vehicle chassis in order to afford the necessary cushioning action. It is also necessary that a seat suspension provide for heightwise adjustability of the seat irrespective of the weight of its occupant, having in mind that a heavy occupant, who tends to depress a resiliently supported seat to a height lower than average, may be a short person for whom the seat should be higher than average. With some of the earlier seat suspension devices, heightwise seat adjustment was not independent of the weight of the occupant, and with others height adjustment involved complicated mechanism or afforded less comfortable riding qualities for seat occupants of certain height/weight ratios than for others.
My copending application Ser. No. 963,376, filed Nov. 24, 1978, discloses a seat suspension arrangement which avoids these problems and disadvantages. The seat suspension of that application comprises a hydraulic cushioning device which is interposed between the seat proper and a chassis member and which comprises a piston that is slidable in a cylinder body both upward and downward from a normal position that it tends to maintain regardless of the weight of the seat occupant. The interior of the cylinder body is at all times communicated through a throttling restriction with a hydraulic accumulator, and therefore the piston can have yielding up and down motion out of its normal position in response to bumps and the like. However, fluid can also flow into and out of the interior of the cylinder body through a pressure fluid port and a vent port, both of which are blocked by a valving portion of the piston when the piston is in its normal position.
The pressure fluid port is connected with a source of hydraulic fluid under substantially constant pressure. By way of the vent port, fluid can leave the interior of the cylinder body through a throttling restriction. If the piston is forced down from its normal position, its valving portion uncovers the pressure fluid port, and hydraulic fluid from the constant pressure source enters the cylinder body, forcing the piston back up to its normal position and increasing the charge on the accumulator to such an extent that the piston tends to be maintained in that position. If the piston rises above its normal position, the valving portion uncovers the vent port, and drainage of fluid out of the cylinder body permits the piston to return to its normal position and effects such partial discharge of the accumulator as tends to maintain it there.
Because the piston always seeks its normal position relative to the cylinder, provision for heightwise adjustment of the seat is external to the cushioning device. Thus, the chassis member on which the device is mounted can be adjustable up and down, or the connection between the seat and the device can be heightwise adjustable.
In a preferred form of the hydraulic cushioning device as disclosed in that application, the cylinder body has a bottom wall, a side wall that projects up from the bottom wall, and a coaxial post-like projection that also projects up from the bottom wall and has a cylindrical surface to which the pressure fluid port and the vent port open. The valving portion of the piston has an internal cylindrical surface which closely surrounds the post portion and which is of such axial extent that it just covers the two ports therein when the piston is in its normal position.
With that arrangement, the stroke of the piston is limited by the axial spacing between the pressure fluid port and the vent port, being, in theory, slightly less than three times the distance between them. The pressure fluid port must be spaced above the vent port by a distance such that both of those ports will just be covered by the valving portion of the piston when the piston is in its normal position. The vent port must be spaced above the bottom wall of the cylinder body by a distance slightly less than the axial length of the valving portion, so that the vent port will be covered by the valving portion of the piston however far below its normal position the piston may be. An upward travel of the piston must be so limited that the pressure fluid port is not uncovered when the piston is at the top of its stroke.
With that arrangement, the piston stroke can theoretically be made as long as desired. For increased stroke length, the vent port is raised farther above the bottom of the cylinder body, the distance between the pressure fluid port and the vent port is increased, and the effective axial length of the valving portion of the piston is correspondingly increased.
As a practical matter, however, the effective stroke length of the piston tends to be limited by the length of the cylinder body, because the whole device must fit in the relatively limited space between the seat proper and the chassis member on which it is mounted. Furthermore, the practical limit of piston travel is actually substantially less than three times the distance between the vent port and the pressure fluid port, because the valving portion of the piston cannot project very far above the top of the post-like portion of the cylinder body when the piston is at the top of its stroke. The reason for the last-mentioned limitation is that the valving portion, although constrained to move axially with the piston proper, must be free for limited radial motion relative to it in order to accommodate any lack of concentricity between the post-like portion and the rest of the cylinder body, and therefore if the valving portion is carried too far up the post-like portion, the valving portion, lacking guidance from the post-like portion, can tilt slightly and bind against it, preventing retraction of the piston. Increasing the height of the post-like portion, to afford more guidance to the valving portion, does not solve the problem because the top of the post-like portion will then engage the end wall of the piston and prevent the piston from retracting completely.
The preferred arrangement disclosed in the copending application also poses difficult manufacturing problems. The ports that open to the cylindrical surface of the post-like cylinder body portion must be communicated with the exterior of the cylinder body through passages which, in part, extend axially through that post-like portion. Since there must be two such passages in the post-like portion, one for each of the ports therein, those passages must be bored with some degree of precision. The longer they have to be, the more difficult and complicated it becomes to keep them straight and accurately located.
With these considerations in mind, the general object of this invention is to provide a hydraulic seat suspension cushioning and shock absorbing device generally like that disclosed in my above-identified copending application, but having a cylinder body that is relatively short in relation to the stroke length of the piston, and having other important advantages that include lower cost and longer useful life.
Another and very important object of this invention is to provide a hydraulic cushioning and shock absorbing device of the general type just described wherein there is a valving element which is separate from the piston, for controlling flow of pressure fluid into the interior of the cylinder and venting of fluid therefrom, and wherein said valving element travels through a stroke which is substantially shorter than that of the piston and in every part of its stroke is in contact all along its length with the post-like portion of the cylinder body.
Affording a stroke for the valving element that is substantially shorter than the piston stroke has very important consequences with respect to the satisfactory functioning of the device and its useful life. In order for the pressure fluid port and the vent port to be completely blocked when the piston is in its normal position, the inner surface of the valving element must obviously have a very close but slidable fit around the cylindrical surface to which those ports open. Although the hydraulic fluid that fills the interior of the cylinder body is a lubricant, wear between such closely fitting sliding surfaces is inevitable, and the greater the amount of relative travel between them, the greater will be the amount of wear. Thus, shortening the stroke of the valving element in relation to the piston stroke brings about a corresponding reduction of the potential for wear on the sealing surfaces.
Perhaps more important from the standpoint of reducing possible wear is that the valving element in the device of this invention never projects above the top of the post-like portion of the cylinder body and therefore, in every position, always receives full guidance from that post-like portion. By thus preventing tilting of the valving element relative to the post-like portion, the end portion of each of those parts is prevented from digging into the sealing surface on the other to score or abrade it.
It will now be apparent that the general object of the present invention is to provide an improved device which is generally like the one disclosed in my copending application but is substantially more compact in having a shorter overall length for a given piston stroke length, is smoother and more reliable in operation, is substantially more durable, and is nevertheless easier to manufacture and therefore lower in cost.